A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps: melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass; forming an article from the glass and cooling the glass below its transformation range; crystallizing ("ceramming") the glass article by an appropriate thermal treatment. The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range, followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei.
Whether a glass-ceramic is transparent or opaque, color may be imparted to it by incorporating one or more colorants, usually transition metal oxides, in the precursor glass. However, the color in a parent glass may change markedly during the ceramming step. Therefore, the ultimate glass-ceramic color is often not predictable from the initial glass color.
U.S. Pat. No. 5,070,045 (Comte et al.) discloses transparent glass-ceramic plates that exhibit minimal distortion when rapidly crystallized from precursor glass plates. The transmission in the infra-red portion of the spectrum is relatively high compared to that in the visible portion. The predominant crystal phase in the glass-ceramics is .beta.-quartz solid solution. Their compositions consist essentially, in weight percent, as calculated on the oxide basis, of:
______________________________________ SiO.sub.2 65-70 MgO + BaO + SrO 1.1-2.3 Al.sub.2 O.sub.3 18-19.8 ZrO.sub.2 1.0-2.5 Li.sub.2 O 2.5-3.8 As.sub.2 O.sub.3 0-1.5 MgO 0.55-1.5 Sb.sub.2 O.sub.3 0-1.5 ZnO 1.2-2.8 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0.5-1.5 TiO.sub.2 1.8-3.2 Na.sub.2 O 0-&lt;1.0 BaO 0-1.4 K.sub.2 O 0-&lt;1.0 SrO 0-1.4 Na.sub.2 O + K.sub.2 O 0-&lt;1.0 BaO + SrO 0.4-1.4 2.8 Li.sub.2 O + 1.2 ZnO &gt;1.8 5.2 MgO ______________________________________
The Comte et al. patent discloses use of 0.1-1.0% of a colorant selected from CoO, NiO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO.sub.2, and V.sub.2 O.sub.5. The presence of the latter (V.sub.2 O.sub.5) is taught to contribute to minimal distortion while giving a black aspect in reflection and a reddish brown tint in transmission.
It is a basic purpose of the present invention to provide an improved means of producing a glass-ceramic material containing V.sub.2 O.sub.5 as a colorant. A particular purpose is to provide a concentrated colorant material to be added to a glass in a forehearth to accomplish this basic purpose. A specific purpose is to provide a convenient and effective way of producing a glass-ceramic, as taught by the Comte et al. patent, that contains V.sub.2 O.sub.5 as an ingredient.
The method and equipment used to add a colorant material to a glass in a forehearth is referred to as a forehearth coloring system or a colorcell. Such systems (colorcells) have been in use for at least 30 years. They have been used primarily, however, to impart colors to soda lime glasses, in particular, green or blue colors to soda lime glass bottles. Currently, colorcells are employed to introduce two types of glass colorants: Unmelted concentrated colorant in particulate form and melted color frits. The former is favored in the United States, while the latter is more popular in Europe.
Because vanadium tends to absorb infra-red rays, it is difficult to obtain even heat distribution in a glass melt having a high V.sub.2 O.sub.5 content. Consequently, our invention is aimed at providing an unmelted concentrated colorant type of material.